1. Field of the Invention
This invention is related to the field of integrated circuit devices and, more particularly, to protection devices of inputs to integrated circuits.
2. Prior Art
In the design of integrated circuit devices, attention has been directed to the protection of the device component inputs disposed for receiving signals from the external world. Compared to the other connections, such as outputs and the ground line, which communicate with the external world, integrated circuit inputs have very small capacitance. Problems arise when externally generated static charges, such as may occur during handling, are transferred to the inputs of the device. Due to the small capacitances in the integrated circuit, these charges can create high voltages in the integrated circuit and wreak havoc among its elements. For example, in MOS integrated circuits the gate of the first MOS transistor receiving an input signal is protected. If not, errant charges can raise the voltage of the gate so high that an electric discharge occurs between the gate and the underlying semiconductor substrate, thereby burning out the gate oxide under the gate creating an incapacitating short circuit.
A standard integrated circuit device has metal contact pads which receive external lead wires joined to the pad by thermal compression or sonic bonding. The metal contact pad is connected to the rest of the integrated circuit by a doped conducting region in the substrate of opposite conductivity to the substrate. The conducting region forms a resistor and a P-N junction diode with the substrate, which in combination serve to release electrostatically generated charges into the substrate, thereby reducing the voltage at the input device for protection thereof.
In spite of this, it has been found that crippling short circuits can also occur at the contact area between the metal from a pad and the doped conducting region before an input device is broken down.
When electrostatic charges discharge through the conducting region, heat is generated which elevates temperatures to the eutectic point of the metal and semiconductor. A metal-semiconductor alloy spike is formed in the conducting region. If the alloy spike reaches the P-N junction boundary, the junction is permanently damaged and the device is rendered useless.